A systematic study was performed on Al2O3 films RF-magnetron sputtered on GaN substrate and subjected to different post-deposition annealing (PDA) temperatures (200-800 degrees C) in oxygen ambient. The as-deposited Al2O3 film and Al2O3 films subjected to PDA at 200 and 400 degrees C were present in amorphous phase and therefore undetectable by X-ray diffraction. By further enhancing the PDA temperature (>= 600 degrees C), a transformation from amorphous to polycrystalline phase of Al2O3 happened. The increment of PDA temperature has contributed to an enhancement in leakage current density-electric field (J-E) characteristics of the investigated samples. A correlation between the acquired J-E characteristics with effective oxide charge, slow trap density, interface trap density, and total interface trap density were discussed. A detailed investigation on the conduction of charges through the as-deposited Al2O3 gate and Al2O3 gates subjected to different PDA temperatures via space-charge-limited conduction, Schottky emission, Poole-Frenkel emission, and Fowler-Nordheim tunneling were presented. (C) 2014 Elsevier B.V. All rights reserved.